The present invention is in the area of chemical vapor deposition, and pertains more particularly to new methods and apparatus for depositing films by atomic layer deposition.
In the manufacture of integrated circuits, deposition of thin films of many pure and compound materials is necessary, and many techniques have been developed to accomplish such depositions. In recent years the dominant technique for deposition of thin films in the art has been chemical vapor deposition (CVD), which has proven to have superior ability to provide uniform even coatings, and to coat relatively conformally into vias and over other high-aspect and uneven features in wafer topology. As device density has continued to increase and geometry has continued to become more complicated, even the superior conformal coating of CVD techniques has been challenged, and new and better techniques are needed.
The approach of a variant of CVD, Atomic Layer Deposition has been considered for improvement in uniformity and conformality, especially for low temperature deposition. However the practical implementation of this technology requires a solution to higher purity and higher throughput. This patent addresses these requirements.
Atomic Layer Deposition
In the field of CVD a process known as Atomic Layer Deposition (ALD) has emerged as a promising candidate to extend the abilities of CVD techniques, and is under rapid development by semiconductor equipment manufacturers to further improve characteristics of chemical vapor deposition. ALD is a process originally termed Atomic Layer Epitaxy, for which a competent reference is: Atomic Layer Epitaxy, edited by T. Suntola and M. Simpson, published by Blackie, Glasgo and London in 1990. This publication is incorporated herein by reference.
Generally ALD is a process wherein conventional CVD processes are divided into single-monolayer deposition steps, wherein each separate deposition step theoretically goes to saturation at a single molecular or atomic monolayer thickness, and self-terminates.
The deposition is the outcome of chemical reactions between reactive molecular precursors and the substrate. In similarity to CVD, elements composing the film are delivered as molecular precursors. The net reaction must deposit the pure desired film and eliminate the xe2x80x9cextraxe2x80x9d atoms that compose the molecular precursors (ligands). In the case of CVD the molecular precursors are fed simultaneously into the CVD reactor. A substrate is kept at temperature that is optimized to promote chemical reaction between the molecular precursors concurrent with efficient desorption of byproducts. Accordingly, the reaction proceeds to deposit the desired pure film.
For ALD applications, the molecular precursors are introduced into the ALD reactor separately. This is practically done by flowing one precursor at a time, i.e. a metal precursor xe2x80x94MLx(M=Al, W, Ta, Si etc.) that contains a metal element xe2x80x94M which is bonded to atomic or molecular ligands xe2x80x94L to make a volatile molecule. The metal precursor reaction is typically followed by inert gas purging to eliminate this precursor from the chamber prior to the separate introduction of the other precursor. An ALD reaction will take place only if the surface is prepared to react directly with the molecular precursor. Accordingly the surface is typically prepared to include hydrogen-containing ligands xe2x80x94AH that are reactive with the metal precursor. Surfacexe2x80x94molecule reactions can proceed to react with all the ligands on the surface and deposit a monolayer of the metal with its passivating ligand: substrate-AH+MLxxe2x86x92substrate-AMLy+HL, where HL is the exchange reaction by-product. During the reaction the initial surface ligands xe2x80x94AH are consumed and the surface becomes covered with L ligands, that cannot further react with the metal precursor xe2x80x94MLx. Therefore, the reaction self-saturates when all the initial ligands are replaced with xe2x80x94MLy species.
After completing the metal precursor reaction the excess precursor is typically removed from the reactor prior to the introduction of another precursor. The second type of precursor is used to restore the surface reactivity towards the metal precursor, i.e. eliminating the L ligands and redepositing AH ligands.
Most ALD processes have been applied to deposit compound films. In this case the second precursor is composed of a desired (usually nonmetallic) element xe2x80x94A (i.e. O, N, S), and hydrogen using, for example H2O, NH3, or H2S. The reaction: xe2x80x94ML+AHzxe2x86x92xe2x80x94Mxe2x80x94AH+HL (for the sake of simplicity the chemical reactions are not balanced) converts the surface back to be AH-covered. The desired additional element xe2x80x94A is deposited and the ligands L are eliminated as volatile by-product. Again, the reaction consumes the reactive sites (this time the L terminated sites) and self-saturates when the reactive sites are entirely depleted.
The sequence of surface reactions that restores the surface to the initial point is called the ALD deposition cycle. Restoration to the initial surface is the keystone of ALD. It implies that films can be layered down in equal metered sequences that are all identical in chemical kinetics, deposition per cycle, composition and thickness. Self-saturating surface reactions make ALD insensitive to transport nonuniformity either from flow engineering or surface topography (i.e. deposition into high aspect ratio structures). Non uniform flux can only result in different completion time at different areas. However, if each of the reactions is allowed to complete on the entire area, the different completion kinetics bear no penalty.
As is often the case with process development, the initial promised advantages of a new technique do not, in the end, attain their full initial promise. Unfortunately, ALD has a serious fundamental problem. Unlike CVD reactions that are of a continuous steady state nature, ALD reactions follow kinetics of molecular-surface interaction. Kinetics of molecular-surface reactions depends on the individual reaction rate between a molecular precursor and a surface reactive site and the number of available reactive sites. As the reaction proceeds to completion, the surface is converted from being reactive to non-reactive. As a result the reaction rate is slowing down during the deposition. In the simplest case the rate, dN/dt is proportional to the number of reactive sites, dN/dt=xe2x88x92kN, where N is the number of reactive sites and k is the (single site) reaction rate. Eliminating reactive sites (or growing of the already-reacted sites) follows an exponential time dependence kN(t)=kN0 exp(xe2x88x92kt). This fundamental property of molecule-surface kinetics was named after the great scientist Langmuir, and is quite well-known in the art.
The interpretation of Langmuirian kinetics limitations illustrates a serious drawback of ALD and a severe deviation from the ideal picture. Accordingly, the self-terminating reactions never really self-terminate (they would require an infinite time because the rate is exponentially decreasing). It means that under practical conditions the surface is never entirely reacted to completion after a deposition cycle. If the surface is not completely reacted there are leftover undesired elements in the film. For example, if the MLx reaction cannot totally consume the surface xe2x80x94AH sites, the film will have H incorporation. Likewise, if the AHy reaction is not carried to completion, undesired L incorporation is inevitable. Clearly, the quality of the film depends on the impurity levels. The throughput-quality tradeoff is of particular concern because it carries an exponential throughput penalty to attain a reduction of impurity levels.
In conventional atomic layer deposition one must accept low throughput to attain high-purity film, or accept lower-purity films for higher throughput. What is clearly needed is an apparatus and methods which not only overcome the Langmuirian limitations but simultaneously provide higher-purity films than have been available in the prior art methods. Such apparatus and methods are provided in embodiments of the present invention, taught in enabling detail below.
In a preferred embodiment of the present invention a method for depositing a metal on a substrate surface in a deposition chamber is provided, comprising steps of (a) depositing a monolayer of metal on the substrate surface by flowing a molecular precursor gas or vapor bearing the metal over a surface of the substrate, the surface saturated by a first reactive species with which the precursor will react by depositing the metal and forming reaction product, leaving a metal surface covered with ligands from the metal precursor and therefore not further reactive with the precursor; (b) terminating flow of the precursor gas or vapor; (c) purging the precursor with inert gas; (d) flowing at least one radical species into the chamber and over the surface, the radical species highly reactive with the surface ligands of the metal precursor layer and eliminating the ligands as reaction product, and also saturating the surface, providing the first reactive species; and (e) repeating the steps in order until a metallic film of desired thickness results.
In many such embodiments the radical species is atomic hydrogen. Using atomic hydrogen a broad variety of pure metals may be deposited, such as tungsten, tantalum, aluminum, titanium, molybdenum, zinc, hafnium, niobium and copper.
In another aspect of the invention a method is provided for depositing a metal oxide on a substrate surface in a deposition chamber, comprising steps of (a) depositing a monolayer of metal on the substrate surface by flowing a metal molecular precursor gas or vapor bearing the metal over a surface of the substrate, the surface saturated by a first reactive species with which the precursor will react by depositing the metal and forming reaction product, leaving a metal surface covered with ligands from the metal precursor and therefore not further reactive with the precursor; (b) terminating flow of the precursor gas or vapor; (c) purging the precursor with inert gas; (d) flowing a first radical species into the chamber and over the surface, the radical species highly reactive with the reaction product and combining with the reaction product to create volatile species and saturate the surface with the first radical species; (e) flowing radical oxygen into the chamber to combine with the metal monolayer deposited in step (a), forming an oxide of the metal; (f) flowing a third radical species into the chamber terminating the surface with the first reactive species in preparation for a next metal deposition step; and (g) repeating the steps in order until a composite film of desired thickness results.
In this method the first and third radical species may be both atomic hydrogen, and the metal surface in step (f) is terminated with hydroxyl species reactive with the metal precursor to deposit the metal. In another embodiment the oxygen and hydrogen atomic steps (e) and (f) are repeated to improve film quality. In still another embodiment steps (e) and (f) are combined into one step wherein the surface is reacted with hydrogen and oxygen atoms simultaneously.
In various embodiments for depositing oxides the oxides can be tantalum pentoxide, aluminum oxide, titanium oxide, niobium pentoxide, zirconium oxide, hafnium oxide, zinc oxide, molybdenum oxide, manganese oxide, tin oxide, indium oxide, tungsten oxide and silicon oxide, among others.
In some embodiments the first radical species is atomic hydrogen and steps (e) and (f) are united to one step using OH radicals, and the metal surface in step (f) is terminated with hydroxyl species reactive with the metal precursor to deposit the metal.
In still another aspect of the invention a method for depositing a metal nitride on a substrate surface in a deposition chamber is provided, comprising steps of (a) depositing a monolayer of metal on the substrate surface by flowing a metal precursor gas or vapor bearing the metal over a surface of the substrate, the surface saturated by a first reactive species with which the precursor will react by depositing the metal and forming reaction product, leaving a metal surface covered with ligands from the metal precursor and therefore not further reactive with the precursor; (b) terminating flow of the precursor gas or vapor; (c) purging the precursor with inert gas; (d) flowing a first radical species into the chamber and over the surface, the atomic species highly reactive with the surface ligands of the metal precursor layer and eliminating the ligands as reaction product and also saturating the surface; (e) flowing radical nitrogen into the chamber to combine with the metal monolayer deposited in step (a), forming a nitride of the metal; (f) flowing a third radical species into the chamber terminating the surface with the first reactive species in preparation for a next metal deposition step; and (g) repeating the steps in order until a composite film of desired thickness results.
In this method the first and third atomic radical species may both be atomic hydrogen, and the metal surface in step (f) may be terminated with amine species reactive with the metal precursor to deposit the metal. Further, steps (e) and (f) may be combined into one step wherein the surface is reacted with hydrogen and nitrogen atoms simultaneously.
In variations of this embodiment a variety of different nitrides may be produces, including, but limited to tungsten nitride, tantalum nitride, aluminum nitride, titanium nitride, silicon nitride and gallium nitride.
In another variation the first radical species may be atomic hydrogen and steps (e) and (f) may be united into one step using one or both of NH and NH2 radicals, and the metal surface in step (f) is terminated with amine species reactive with the metal precursor to deposit the metal.
In yet another aspect of the invention a process for building a metal, metal oxide, or metal nitride film on a substrate surface is provided, wherein deposition steps comprise flowing a metal precursor gas or vapor over the surface with the surface terminated with a first chemical species reactive with the metal precursor to deposit the metal, are alternated with steps comprising flowing radical species over the freshly deposited metal layers to remove the ligands from the deposition steps and to provide the first chemical species to terminate the substrate surface preparatory to the next deposition reaction.
In this process a metal nitride film is built up by a step sequence of metal deposition by reacting a metal precursor gas with a surface terminated by amine species, then alternating exposure of the surface with atomic radical hydrogen, nitrogen and hydrogen again, thereby volatilizing products remaining from the metal deposition chemistry, nitridizing the deposited metal monolayer, then terminating the metal surface with amine species again in preparation for a next metal deposition step. A metal oxide film is built up by a step sequence of metal deposition by reacting a metal precursor gas with a surface terminated by hydroxyl species, then alternating exposure of the surface with atomic radical hydrogen, oxygen and hydrogen again, thereby volatilizing products remaining from the metal deposition chemistry, oxidizing the metal monolayer, then terminating the metal surface with hydroxyl species again in preparation for a next metal deposition step.
In yet another aspect of the invention a method for depositing a compound film on a substrate surface in a deposition chamber is provided, comprising steps of (a) depositing a monolayer of metal on the substrate surface by flowing a metal molecular precursor gas or vapor bearing the metal over a surface of the substrate, the surface saturated by a first reactive species with which the precursor will react by depositing the metal and forming reaction product, leaving a metal surface covered with ligands from the metal precursor and therefore not further reactive with the precursor; (b) terminating flow of the precursor gas or vapor; (c) purging the precursor with inert gas; (d) flowing a first radical species into the chamber and over the surface, the radical species highly reactive with the reaction product and combining with the reaction product to create volatile species and saturate the surface with the first radical species; (e) flowing nonmetal atomic species into the chamber to combine with the metal monolayer deposited in step (a), forming a compound film of the metal; (f) flowing a third radical species into the chamber terminating the surface with the first reactive species in preparation for a next metal deposition step; and (g) repeating the steps in order until a composite film of desired thickness results.
In this method the first and third radical species may be both atomic hydrogen, and the metal surface in step (f) is terminated with hydride species of the nonmetallic element that are reactive with the metal precursor to deposit the metal. In a variation the non-metallic and hydrogen atomic steps (e) and (f) are repeated to improve the film quality. In another variation steps (e) and (f) are combined into one step wherein the surface is reacted with hydrogen and non-metallic atoms simultaneously. A variety of films may be produced by practicing this variation of the invention as well, including but not limited to molybdenum disulfide and zinc sulfide.
In yet another aspect of the invention a radical-assisted sequential CVD) (RAS-CVD) reactor is provided, comprising a chamber with controlled gas inlets for introducing gases in sequential steps and a heated substrate support for holding a substrate and exposing a surface of the substrate to incoming gases; and a plasma generation apparatus for generating radical atomic species for use in the reactor. In this reactor an aggregate metal layer is formed by depositing a monolayer of metal on the substrate surface by flowing a precursor gas or vapor bearing the metal over a surface of the substrate, the surface terminated by a first reactive species with which the precursor will react by depositing the metal and forming reaction product, leaving a metal surface not further reactive with the precursor, terminating flow of the precursor gas or vapor, flowing at least one atomic radical species into the chamber and over the surface, the atomic species highly reactive with the reaction product and combining with the reaction product, and also terminating the surface, providing the first reactive species, and repeating the steps in order until a composite film of desired thickness results.
In various embodiments the atomic radical species is atomic hydrogen. The precursor gas bearing the metal may be tungsten hexafluoride and the metal deposited tungsten.
In some embodiments the plasma generation apparatus comprises an electrode within the reactor chamber and a high frequency power supply connected to the electrode. In other embodiments the plasma generation apparatus comprises a showerhead-type gas distribution apparatus, and a plasma is generated within the showerhead apparatus to produce the radical species. In still other embodiments the atomic radical species is produced in a remote plasma generator, and the species are delivered to the reactor.
In the various embodiments of the invention a new process is provided wherein films of many sorts, including pure metals, oxides of metals, nitrides of metals, and other films, may be produced quickly and efficiently, with very high purity and with superior conformity to substrate topography and coverage within vias and other difficult surface geometries.